An ice pack will work for a day or two, depending on the temperature of the truck it is in. In the winter you might not need one. In the summer you might get fast enough shipping that the ice pack will do, maybe not. I have had bad experiences with shipping liquid yeast so I will only get it from my local store, where I know it has been handled well. I shop at the Los Altos MoreBeer, which gets a truck delivery from their warehouse every Friday. It is only a few hour drive and everything that needs to be refrigerated is kept cold during the trip. I have never had a problem with the liquid yeast I get from them.